The invention pertains to a magnetic resonance imaging system comprising an excitation antennae system including several antennae for emitting an RF-excitation field [B1(t)], an activation control unit coupled to the excitation antennae system and arranged for activating the excitation antennae system.
Such a magnetic resonance imaging system is known from the paper xe2x80x98A k-space analysis of small-tip-angle excitationxe2x80x99 by John Pauly et al. in the Journal of Magnetic Resonance 81(1989)43-56.
The cited reference proposes to employ spatially selective RF-excitations in the magnetic resonance imaging system. Such spatially selective RF-excitations are achieved by scanning the applied RF-energy across k-space. Notably, the cited reference shows that inherent focussing of slice selection RF-excitations is achieved and also that spatially selective RF-excitations in two-dimensions are achieved.
However, the scanning of k-space as proposed in the cited reference leads to the need of applying complicated and lengthy sequences of temporary magnetic gradient fields and RF-excitation pulses. Consequently, the known spatially selective RF-excitations are time consuming and impede the formation of magnetic resonance images in rapid succession.
An object of the invention is to provide a magnetic resonance imaging system which requires less time to achieve the RF-excitation. In particular, an object of the invention is to reduce the time required for spatially selective RF-excitations.
This object is achieved by the magnetic resonance imaging system according to the invention wherein the activation control system is arranged such that: individual antennae are activated to simultaneously emit separate RF-excitation constituents [Bn(t)]; and RF-excitation constituents having different activation distributions over k-space.
The magnetic resonance imaging system according to the invention produces magnetic resonance signals from which a magnetic resonance image is reconstructed. The magnetic resonance imaging system comprises a magnet system which provides a substantially uniform main magnetic field. The object to be examined is placed in the main magnetic field. The magnetic resonance signals are generated upon the RF-excitation. The RF-excitation causes magnetic spins in the object to be examined to be excited and subsequently these magnetic spins relax while emitting the magnetic resonance signals. The magnetic resonance signals are spatially encoded by application of temporary magnetic gradients, notably the so-called phase-encoding gradients and read-gradients provide the spatial encoding of the magnetic resonance signals. Also temporary magnetic gradient fields may be applied during the RF-excitation so as to bring about the scanning the applied RF-energy across k-space i.e. scanning the wavevector of the RF-excitation. The temporary magnetic gradient fields, also indicated as gradient pulses, are superposed on the main magnetic field applied by the magnet system. Usually gradient coils are employed to provide the gradient pulses. The RF-excitations are applied by the antennae system having one or several antennae. Preferably, RF-coils are employed as these antennae.
The activation unit of the magnetic resonance imaging system according to the invention performs a decomposition of the complete time-varying magnetic excitation field into several RF-excitation constituents. Separate RF-excitation constituents are emitted by respective antennae or RF-coils simultaneously. Thus it is achieved to perform the RF-excitation to a large degree in parallel. The degree of parallelism involved depends on the number of RF-excitation constituents employed. Consequently, the time required to apply the RF-excitation is accordingly reduced. Notably, the magnetic resonance imaging system of the invention makes more efficient use of the time available for the RF-excitation so that less time is needed to perform the excitation or a more complicated RF-excitation may be employed still within an acceptable period of time. The magnetic resonance imaging system according to the invention is suitable in particular for RF excitation of regions of intricate shapes.
These and other aspects of the invention will be further elaborated with reference to the embodiments defined in the dependent claims.
Preferably, the RF-excitation constituents contribute to the RF-excitation field according to the decomposition into harmonic components of the spatial RF-emission profiles of the individual antennae.
According to the invention, spatial encoding is employed by both gradient encoding and encoding on the basis of the spatial RF-emission profile of the RF-excitation coil. Thus, the excitation requires a more simple excitation sequence which is relatively short and achieves an accurate excitation of the required region. In practice this is achieved as follows:
1. Determine the spatial region to be excited.
2. Deform this region into xe2x80x98foldedxe2x80x99 regions in which pixels are superpositions of pixels i.e. local RF-excitations levels, of the original spatial region to be excited and which superposition is encoded (weighted) on the basis of the spatial RF-emission profiles of the excitation antennae. These xe2x80x98foldedxe2x80x99 regions form aliased spatial RF-emission profiles for the individual antennae. The superpositions are such that upon combination, e.g. adding, of the folded regions, overall RF-excitations due to the combined RF-excitation constituents cancel except in the pre-determined spatial region.
3. Perform a Fast Fourier transform to the folded region to derive the excitation waveforms which forming the RF-excitations constituents for each of the RF-coils involved together with the gradient pulses that cause the scanning across k-space.
Note that simultaneously applied RF-excitation constituents by excited RF-coils are activated while the same gradient pulse are applied. Hence, no additional time is required to apply different gradient pulses during simultaneous RF-excitation constituents. Simultaneous RF-excitation constituents are associated with the same traversal through k-space, i.e. contribute to the k-space trajectories simultaneously, but with possibly different amplitudes.
One embodiment of the magnetic resonance imaging system of the invention employs RF-excitation constituents each having different supports on k-space. The support on k-space of this RF-excitation constituent is the set of wavevectors (k-vector values) for which the RF-excitation constituent has a non-zero complex value. This embodiment employs excitation antennae, notably RF-coils, having a spatial sinusoidal spatial RF-emission profile.
In a preferred embodiment of a magnetic resonance imaging system according to the invention, the RF-excitation constituents have activation distributions over k-space that cause a sub-scanning of k-space along a sub-scanning direction. The term sub-scanning indicates that a less dense scanning of k-space than what is required in view of the spatial resolution of the spatial region to be excited, is carried-out by the RF-excitation constituent at issue. The computation of the RF-excitation constituents involves matrix inversion that is more stable as the predominant spatial variation of the spatial RF-emission profiles of the excitation antennae is along the sub-scanning direction.
Preferably, RF-surface coils are employed as the antennae of which the spatial RF-emission profile has emission phase changes mainly in the plane of the RF-surface coils. The RF-surface coils are substantially planar. Preferably respective RF-surface coils have their planes separated along the read-direction. In this formation, the RF-surface coil emission phases varies predominantly in the phase-encoding direction, viz. parallel to the plane of the RF-surface coil and the emission amplitude varies mainly along the read-direction transversely to the RF-surface coil plane. In this situation the computation of the RF-excitation constituents in particular for Cartesian trajectories involves a more stable matrix inversion. The matrix inversion can be stabilised by using adequate k-space trajectories and adequately designed and positioned coils. As a general rule, the trajectories should cover the k-space as smoothly as possible, and the coils should have (as good as possible) sinusoidal phase in the sub-scanning direction in which the density of scanned positions in k-space is lower, i.e., in the phase-encoding or preparation direction for Cartesian, in radial direction for spiral, and in tangential direction for radial trajectories.
The invention further relates to a magnetic resonance imaging method. The magnetic resonance imaging method according to the invention makes more efficient use of the time available for the RF-excitation so that less time is needed to perform the excitation or a more complicated RF-excitation may be employed still within an acceptable period of time. The magnetic resonance imaging method according to the invention is in particular well employed to perform intricate shaped spatially shaped regions.
The invention further relates to a computer programme. The computer programme is in particular suitable to carry out the magnetic resonance imaging method of the invention. Further, a magnetic resonance imaging system according to the invention can be provided by loading the computer programme according to the invention into the working memory of a computer incorporated in a general conventional magnetic resonance imaging system. The computer programme can be provided on a data carrier, such as a CD-rom and the computer programme can also be provided via a data network, such as the world-wide web, via which data network the computer programme according to the invention can be downloaded into the working memory of the computer of the magnetic resonance imaging system.